1. Field of the Invention
This invention relates to sorbents for the removal of acid anhydrides from effluent gases, and more specifically, this invention relates to two classes of amine-based sorbents: polymer-immobilized tertiary amines and polymer-bound tertiary amines which can serve as sorbents for the removal of carbon dioxide and other acid anhydrides from any effluent stream.
2. Background of the Invention
Carbon dioxide is considered to be the major greenhouse gas due to its effect on the earth's ozone layer. It has been estimated that 36% of the United States' anthropogenic CO2 is produced from coal-fired power plants. Consequently, the capture and sequestration of CO2 from flue gas streams is an essential step for carbon management in our environment.
New research efforts for the capture and sequestration of carbon dioxide (CO2) from flue gas streams are being proposed and will have a direct or indirect impact on the types of systems ultimately designed and implemented. Currently, techniques for the capture and separation of CO2 employ solvents, cryogenic techniques, membranes, and solid sorbents. Large-scale operation of these technologies is energy intensive when applied to capturing CO2 in dilute streams, such as flue gas. For example, coal combustion gases comprise approximately 15 mol % CO2.
Carbon dioxide is removed from natural gas streams via stripping into aqueous amine solutions. A significant improvement over the past 30 years has been the introduction of sterically hindered amines which allow for a 1:1 molar adsorption ratio of CO2 to amine rather than the 1:2 ratio found in typical primary and secondary amines. The most serious drawbacks to aqueous amine processes are corrosion of process vessels, degradation of amine during thermal cycling, and the large energy requirements owing to water's high heat capacity.
Amine-based wet scrubbing systems have also been proposed as capture techniques for CO2 removal from flue gas streams. However, these systems are very energy-intensive due to the large amount of water needed and the need to recover the amines via evaporation of the water. Large amounts of water are required because of the mechanism, corrosiveness, and air flow problems created by the use of monoethanolamine (MEA), diethanolamine (DEA), or methyldiethanolamine (MDEA). Also, these amines are volatile and losses can occur due to evaporation. Thus, these amines lack thermal stability.
FIG. 1 displays the reaction sequences in aqueous systems for primary and secondary amines when reacting with dissolved CO2. As shown in FIG. 1, the majority of the CO2 captured in liquid amine capture systems will result in the formation of a carbamate. In aqueous media, there exists a stoichiometric requirement of 2 moles (mols) of amine per mol of CO2. However, the use of 1,8 Diazabicyclo-[5.4.0]-undec-7-ene (DBU), which has imine functionality (R1N═R2R3, wherein R represents an alkyl group), can reduce the stoichiometric amount of amine required for capture of CO2 to a 1:1 molar ratio, and can do so reversibly. This is shown in D. J. Heldebrant, P. G. Jessop, C. A. Thomas, C. A. Eckert, and C. L. Liotta, “The Reaction of 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) with Carbon Dioxide,” J. Org. Chem., 70, 5335-5338 (2005). This 1:1 molar ratio can double the CO2 loading capacity of any of the currently known CO2 sorbents.
FIG. 2 displays the stoichiometry of the reaction between DBU and carbon dioxide. As noted, the presence of water is important for the formation of carbonate. This is shown in H. Y. Huang, R. T. Yang, D. Chinn, and C. L. Munson, “Amine-Grafted MCM-48 and Silica Xerogel as Superior Sorbents for Acidic Gas Removal from Natural Gas,” Ind. Eng. Res., 42, (12), 2427-2433 (2003). In that work, primary amines were bonded to inert supports and absorbed 5 wt. % CO2 in the presence of a 5 mol % CO2 gaseous mixture. In the presence of 1 atmosphere (atm) of CO2, that percentage increased to 9 wt. %. Temperature Programmed Desorption (TPD) and Mass Spectrometry (MS) analyses were also performed.
Presently, immobilized primary and secondary amine sorbents are utilized in aircraft, submarine, and spacecraft technologies. The immobilization of these amines on supports, via evaporation of the amines into the supports' pores, increases their thermal stability and thus reduces evaporation losses. Nevertheless, the cost of these sorbents is too high for large-scale applications in utility industries such as electrical generation, in part because these sorbents adsorb acid anhydrides at a 2:1 molar ratio of amine to anhydride. Also, these sorbents operate only at 25° C. and are thus limited for other applications.
Some work has been done in attempting to improve upon current sorbents' limitations by the preparation of less costly immobilized secondary amine sorbents, as reported in M. L. Gray, Y. Soong, K. J. Champagne, H. Pennline, J. P. Baltrus, R. W. Stevens, Jr., R. Khatri, S. S. C. Chuang, and T. Filburn, “Improved Immobilized Carbon Dioxide Capture Sorbents,” Fuel Processing Technology, 86 (14), 1449-1455 (2005). While the sorbents seemed to have a stronger adsorption capability for CO2, their loading capacity decreased with each adsorption/regeneration cycle.
U.S. Pat. No. 6,908,497 awarded to Sirwardane on Jun. 21, 2005 discloses carbon dioxide-specific sorbents fabricated by placing amines and/or ethers between the unit layers of an inert substrate's lattice.
U.S. Pat. No. 6,582,498 awarded to Sass, et al. on Jun. 24, 2003 discloses a method of separating carbon dioxide from a gas using a fluid dynamic instability. The method includes the use of a tertiary amine, methyldiethanolamine (MDEA).
U.S. Pat. No. 6,547,854 awarded to Gray, et al. on Apr. 15, 2003 discloses carbon dioxide-specific sorbents fabricated by chemically bonding amines to inert substrates.
U.S. Pat. Nos. 6,364,938; 5,876,488; and 5,492,683 awarded to Birbara, et al. on Apr. 2, 2002; Mar. 2, 1999; and Feb. 20, 1996, respectively, disclose carbon dioxide-specific sorbents fabricated by chemically bonding amines to inert and polymeric substrates.
None of the aforementioned patents discloses a flue gas stream sorbent which can adsorb CO2 at a 1:1 molar ratio of amine to CO2.
None of the aforementioned patents and articles discloses an immobilized tertiary amine-based sorbent which can adsorb carbon dioxide at temperatures in excess of 25° C.
None of the aforementioned patents and articles discloses a chemically bound tertiary amine-based sorbent which can adsorb carbon dioxide at temperatures in excess of 25° C.
In addition, none of the aforementioned patents disclose a flue gas stream sorbent for acid anhydrides which can be used in a system open to the environment.
A need exists in the art for sorbents which can adsorb CO2 at higher CO2 to sorbent molar ratios. A need also exists in the art for acid anhydride sorbents with greater thermal stability that can thus adsorb CO2 at temperatures significantly above 25° C. Finally, a need exists in the art for acid anhydride sorbents which can be used in open systems.